1. Field of Invention PA1 2. Description of the Prior Art
The present invention relates to a system for acquiring and demodulating burst of PN coded data in near real time. More particularly, the present invention relates to a system that is adapted to receive burst transmission of spread spectrum data and rapidly acquire and track the data when known to be in a predetermined window of uncertainty employing a novel parallel despreader.
Heretofore analog and digital acquisition and demodulation systems were known. In our U.S. Pat. Nos. 5,101,370 and 5,099,494 there is shown and described a digital accumulator and scaler and a six channel digital demodulator used in digital acquisitions systems. These circuits were employed to search a maximum window of uncertainty and to acquire (lock-on) and track a received PN code, preferably in a quadrature phase shift keyed (QPSK) format, and further employed a real and imaginary channel for receiving data.
During a search sequence, the PN code was incrementally shifted one-half or at most one chip to acquire the incoming code. The acquisition and demodulation of the incoming signals were recovered using a fixed or variable dwell time before slipping the PN code in an attempt to achieve lock-on of the replica PN code. It is known that variable dwell time will produce faster acquisition than fixed dwell time by quickly rejecting the wrong signal. Heretofore, it was known that correlators could be employed to speed up acquisition of a received code. There is a present hardware limitation as to the number of stages that such correlators may employ before becoming impractical or inefficient for use in a high processing gain acquisition system. It is known that high processing gain acquisition systems employ a very large number of chips which is not compatible with use of a correlator.
The above acquisition system assumes that a replica PN code acquires (locks on) the received signal. When the received signal is a burst of information of known length without a synchronizing header it is virtually impossible to acquire (lock on) to the received signal.
It would be desirable to provide an asynchronous acquisition and demodulation system for spread spectrum codes which employ a highly efficient parallel correlator that may be used with high processing gain acquisition systems to process burst of information without a header.